Just War Theory

With constant international turmoil, escalating Geo-Political issues, quarrels based on egotistical schoolboy dick-measuring, and the ever-present threat of nuclear war, one must ask themselves, ‘What is a Just War?’

Is there such a thing? The history of the concept starts here…




The Western philosophical concept, jus bellum iustum (Just War) is credited to the African Father, Saint Augustine of Hippo (354-430 AD). A key element to Augustine’s Just War Theory was Christian insight, learnt from Aurelius Ambrosius, later known as St Ambrose (340-397 AD).

Ambrose taught Augustine the plenary of the gospel and was responsible for the majority of his theological ambitions, including his seminal work in De Civitate Dei (City of God).

Ambrose provided theological insight but it was Cicero (106-43 BC) who provided particular inspiration with the original concept of jus gentium (Law of Nations).  Cicero’s views on the importance of the preservation of the State, impunity for agents of the State (soldiers), and the importance of maintaining good faith with the enemy, assisted Augustine to expand on his theories. Augustine’s opinions on the downfall of Rome, his ideas on pre-Christian human legal positivism, and on the natural and eternal laws, were all conducive to his philosophy.

Thomas Aquinas (1225-1274) later added to Augustine’s Just War Theory in his Summa Theologica. The gradual shift from moral accountability on the individual, to that of the State, (additions by Thomas Hobbs, 1588-1679) created a divergence from jus ad bellum (Right to War) to jus in bello (Right in War). Others such as Martin Luther (1483-1586) also followed a line of inquiry that led to a division between individual and State. The distinction between the right to start a war, and the rights in war, was the beginning of the doctrine developed by Hugo Grotius (1583-1645). Grotius defined what constituted jus in bello and further developed Augustine’s Just War Theory to include Cicero’s jus gentium. The succession of notable philosophers, along with the eventual Grotian doctrine, coexisting with Thirty Years War that lead to the Treaty of Westphalia—was the dawn of what is now known, as Public International Law.




Augustine attributed the decline of Rome to a lack of virtue. He lived through the period of decline and it had a fundamental influence on his philosophy.[2] Augustine held that a just war could only be established under Christian dogma within a legitimate State.[3]Only a State under God—part of the divine eternal order, could a State ever truly be legitimate: humans serving natural law thereby serving the eternal law.[4] He later affirmed, ‘[n]othing in the temporal law [man made law] is just and legitimate which human beings have not derived from the eternal law’[5]. Augustine recognized a sharp division between pre-Christian positivist human law and natural law of a teleological nature brought about by the Stoics.[6]The recognition of this division was the beginning of his catechizing of the concept of eternal law.[7]

Written shortly after the sacking of Rome by the barbarians (AD 413-426), his work in De Civitate Dei suggests that in its earlier years, Rome possessed the requisite degree of virtue and justice which allowed it to endure. [8]  Augustine connected Rome’s downfall to the State’s subsequent national pride and desire for imperial glory, rather than the love of God, which was required to display true virtue.[9]  Thomas Aquinas expanded on the concept of virtue, detailing the fourfold division of personal virtue: cardinal virtues of prudence, justice, courage and temperateness.[10] These virtues were extracts from the Neoplatonist forms of earthly love: health, pleasure, honor and wealth.[11] Aquinas criticizes the temporal states of love as they are exactly that, and are subject to the fallacy of human will.[12] He asserts the forms of love are vain and selfish, and criticizes all forms of earthly love, including the pursuit of virtue itself. His argument was that the quest for attainment of genuine virtue is based on pride, turning the pursuit into a reckoned vice.[13]Augustine made a distinction between love for things that, against our will, can be taken away from us (earthly love), to the ‘right love,’ which is the love commanded by God.[14] Aquinas continued this inquiry in that, the only true virtue comes from God and that where there is no true religion, there can be no true virtue[15]. The cardinal virtues of courage, temperance, prudence and justice are moral virtues; according to Aquinas, a moral virtue without charity lacks the very essence of virtue[16].



Charity is essential for virtue but it could also force a person to submit to an aggressor’s violence, to protect one’s soul from the disordered passions of vengefulness and hatred.[17] In Summa Theologica, Aquinas discusses just warfare, making a distinction between cardinal virtues, and the importance of charity in giving those virtues their life and existence.[18] He later suggested that waging a just war could be virtuous, but only after the participant developed two natural dispositions in relation to warfare: military prudence and battlefield courage.[19]These dispositions would apply to soldiers and those who would maintain public order under an evangelical command.[20]The moral authority to kill in battle was given by public authority (Princes) and justified by St. Paul’s Epistle to the Romans.[21]He noted a distinction between public and private good, in that, ‘[i]s more apt to conserve human society than removing from man the power of causing harm in private.’[22]Removing this power from man enhanced the moral authority for a just war, if it was sanctioned by a State under Augustine’s view of eternal law.[23] However, a dilemma for Aquinas was Matthew 5:39: ‘But I tell you, do not resist an evil person. If anyone slaps you on the right cheek, turn to them the other cheek also.’ Originally, his interpretation of this passage was that Christian life was incompatible with military service.[24] However he later accepted a different approach, one gathered from from St Ambrose in his De Officiis Ministrorum (Duties of Ministers): ‘The courage whereby a man in battle defends his country against barbarians, or protects the weak at home, or his friends against robbers is full of justice …’[25] Augustine further refined this issue in De Civitate Dei, ‘[h]e to whom authority is delegated, and who is but the sword in the hand of him who uses it, is not himself responsible for the death he deals.’[26]This stark division between individual soldiers and the State was acknowledged to by Hobbes, many years later in De Cive (On the Citizen) in 1647:

What I beleeve to be another mans sin, I may sometimes doe that without any sin of mine. For if I be commanded to doe that which is a sin in him who commands me, if I doe it, and he that commands me be by Right, Lord over me, I sinne not; if I wage warre at the Commandement of my Prince, conceiving the warre to be unjustly undertaken, I doe not therefore doe unjustly.[27]



Augustine determined three criteria that need to be met for a just war: authority, intention and necessity.[28] Firstly, arms may only be taken against the enemy when there is a constituted legal authority that is sanctioned by God.[29]There must be a rightful intention; wars are to be considered evil when they are waged from earthly loves such as cruelty, violence, wild resistance and lust for power.[30]Thirdly, there must be a necessity to punish injustice and preserve peace, declared expressly in his letter to Roman General Bonifacius: ‘Peace should be the object of your desire; war should only be waged as a necessity, and waged only that God may deliver men from the necessity and preserve them in peace’.[31]

Through a process of amelioration, Augustine’s Just War Theory became a critical philosophical antecedent to the development of modern day public international law. A retrospective view on the theory suggests it was almost destined to happen. Starting with the shift from classical Just War Theory, with its emphasis on the individual to a more traditional view with the sovereign State as the principal agent in war.[32]Rousseau declared: ‘[w]ar…is something that occurs not between man and man, but between States. The individuals who become involved in it are enemies only by accident…A State can have as its enemies only other States, not men at all.’[33]Hobbs later diverged from the emphasis on the individual, to that of the State—his pragmatic moral and political theory played a decisive role in the shift from jus ad bellum (Right to War) to jus in bello (Right in War). The shift from jus ad bellum to jus in bello, re-defined the soldier: those who fight for an unjust cause and those that fight for a just cause are moral equals, as long as the rules of proportionality and discrimination (jus in bello) are obeyed—both soldiers may kill and permissibly be killed.[34] Dutch scholar Hugo Grotius (1583-1645) is known for liberating International law from theology and drawing heavily from Augustine’s history of ideas.[35]It has been suggested that the Christian Just War Theory attributed to the African Father (Augustine), and later, Aquinas, has been all but neglected in the development of the modern day doctrine. Prior to the end of the high Middle Ages, the doctrine encompassing jus ad bellum declined significantly in further theories of war.[36]Martin Luther (1483-1586) believed that the Church and State should operate on entirely different principles, leading to the Anabaptist hermeneutic.[37]Hugo Grotius (1583-1645) followed this line of inquiry, asserting that a sovereign State may resort to war if the reason for doing so, was under natural law, and was based on a just cause. He described a just cause as defence against injuries, recovery of what was legally due or the infliction of punishment on a wrongdoing State for excessive crimes.[38]


In his treatise de iure ac pacis (The Law of War and Peace) Grotius notes that there exists a tacit agreement among sovereign States, that belligerents in a public war may, with impunity, commit acts which would be impermissible under natural law.[39]The idea of a jus gentium (The law of Nations) was first developed by Cicero as a mid-point between positivist and natural law—existing as a third form of law between State-centric laws and the law of the cosmos.[40]After the religious intolerance in the Thirty Years War (1618-1648), the concept of jus gentium came to fruition with the Treaty of Westphalia (1648).[41]  Following on from the Treaty of Westphalia, the League of Nations (1920) is a much later example of member States agreeing upon commitments to certain fields, including jus in bello, with an undertaking that international law should be the actual rule of conduct among governments.[42] If the modern day United Nations is seen as successful in the adoption of a bill on the fundamental rights of man, then they have accomplished a task that originated with the international basis in the Treaty of Westphalia.[43]On the beginnings of international law, Grotius, ‘[d]eveloped a system of International Law, which would equally appeal to, and be approved by, the believers and the atheists, and which would apply to all States irrespective of the character and dignity of their rulers…’[44]  The Congress of Westphalia led various States to adopt the principle of societas gentium (Society of Nations), but it was the work of Hugo Grotius that propelled the concept of the jus gentium (Law of Nations) into the future.

Modern International Public law would not be the same without counsel from a long line of philosophical thinkers. Concepts such as St Augustine’s Jus Bellum Iustum in City of God started a philosophical battle amongst other great thinkers, in the role of theology in the justification for warfare. Aquinas’ contribution to the Just War Theory continued with Godly love and the need for virtue in warfare. The shift from jus ad bellum to jus in bello was a turning point in the eventual formulation of rules of war. Hobbes’ work in De Cive and others such as Rousseau, highlighted that the distinction between the soldier and the State, was crucial in the ultimate progression from Cicero’s jus gentium to concepts in Grotius’ de iure ac pacis. Once the distinction was affirmed, philosophers looked at the developments in the rules of war rather than what made a war just. Grotius not only took from Cicero the idea of impunity for soldiers acting under a delegated authority, he embraced Cicero’s Law of Nations and developed his own ideas which were influential in the Treaty of Westphalia—arguably the original source of international law. Grotius’ de iure ac pacis solidified him as a luminary in the development of international law but only because Augustine, Aquinas, Cicero and others, all had shoulders wide enough in which to place a firm footing.

[1] Nico Vorster, ‘Just War and Virtue: Revisiting Augustine and Thomas Aquinas’ (2015) 34(1) South African Journal of Philosophy, 56

[2] Ibid.

[3] Jean Elshtain, Just War Theory (New York University Press, 1992) 9

[4] Anton Chroust, ‘The Philosophy of Law of St. Augustine’ (1944) The Philosophical Review 53(2) 197

[5] Peter King, Augustine: On the free choice of the will, on grace and free choice and other writings (Cambridge University Press, 2010) 13

[6] Brad Inwood, Ethics and Human Action in Early Stoicism (Oxford: Clarendon Press, 1985) 193

[7] Ibid.

[8] Vorster, above n 1, 59.

[9] Vorster, above n 1, 57.

[10] Thomas Aquinas, Jeffrey Hause and Claudia Eisen Murphy, Disputed Questions on Virtue (Hackett Publishing, 2010) 255

[11] Ibid.

[12] Ibid 256.

[13] Ibid 257.

[14] Gareth B Matthews, The Augustinian Tradition (University of California Press, 1999) 333.

[15] Ibid.

[16] Vorster, above n 1, 11.

[17] Gregory M. Reichberg, ‘Thomas Aquinas between Just War and Pacifism’ (2010) 38 Journal of Religious Ethics 225.

[18] Aquinas, above n 9, 249.

[19] Reichberg, above n 16, 235.

[20] Ibid 231.

[21] Ibid.

[22] Ibid.

[23] Elshtain, above n 3.

[24] Reichberg, above n 16, 237.

[25]  Ambrosius and Ivor J Davidson, De Officiis (Clarendon Press, 2001).

[26] Jeff McMahan, ‘War’ (2012) The Oxford Handbook of Political Philosophy quoting Augustine, Marcus Dods and Thomas Merton, The City of God (Modern Library, 1950) 27.

[27] Jeff McMahan, ‘War’ (2012) The Oxford Handbook of Political Philosophy quoting Thomas Hobbes, Elementa Philosophica De Cive (Apud Ludovicum Elzevirium, 1647).

[28] Vorster, above n 1, 58.

[29] Ibid.

[30] Ibid.

[31] Nico Vorster, ‘Just War and Virtue: Revisiting Augustine and Thomas Aquinas’ (2015) 34(1) South African Journal of Philosophy, quoting Philip Schaff and Henry Wace, ‘A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church’ (1890) Parker 1(2) 552-554

[32] McMahan, above n 19, 300.

[33] Ernest Barker et al, Social Contract (Oxford University Press, 1960) 249-50.

[34] B.J Strawser, ‘Walking the Tightrope of Just War’ (2011) 71 Analysis 533.

[35] Phillip Gerald Wynn, Augustine on War and Military Service (Fortress Press, 2013) 12.

[36] Above, n 27, 24.

[37] Jeremy Seth Geddert, ‘David D. Corey and J. Daryl Charles, The Just War Tradition: An Introduction’ (2014) 51 2 Society 189.

[38] Sean D. Murphy. ‘Jus Ad Bellum, Values, and the Contemporary Structure of International Law’ (2013) 41 Journal of Religious Ethics 21.

[39] Gregory M. Reichberg, ‘The Moral Equality of Combatants—A Doctrine in Classical Just War Theory? A Response to Graham Parsons’ (2013) 12 Journal of Military Ethics 186.  

[40] William E. Conklin, ‘The Myth of Primordialism in Cicero’s Theory of Jus Gentium’ (2010) 23 Leiden Journal of International Law 481.

[41] Leo Gross, ‘The Peace of Westphalia, 1648-1948’ (1948) 42 The American Journal of International Law 20.

[42] Ibid 21.

[43] Ibid 24.

[44] Ibid 26.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Create a website or blog at WordPress.com

Up ↑

%d bloggers like this: